Issue due to locale-specific translation of character strings with `strptime`

As reported in thread 138 of the Coursera/rprog-007 discussion forums, `strptime` translates character strings according to the locale:

```
# In an English locale the following returns NA, but in a Spanish locale it works:
strptime("Octubre 17, 1986", "%B %d, %Y")

# In a Spanish locale the following returns NA, but in an English locale it works:
strptime("October 17, 1986", "%B %d, %Y")
```
